1. Field of the Invention
The present invention generally relates to a method for manufacturing a pigment dispersion. The present invention also relates to the manufactured pigment dispersion itself, and a recording liquid for inkjet printing that uses the pigment dispersion.
2. Description of the Related Art
Significant progress has recently been achieved in the field of digital printing technologies represented by electrophotography and inkjet technology, and these technologies have been finding ever growing application as image forming technologies in offices and homes. Water-soluble dye inks have been widely used as printing inks, but issues of blurring, feathering, and weather resistance may be associated therewith. The use of pigment inks has been studied in recent years as an improvement thereto, and inks for inkjet printing containing a pigment dispersion in an ink composition have started finding application. However, pigment particles present in pigment inks can, in some instances, cause light scattering and light reflection. As a result, images formed by pigment inks typically tend to have a color developing ability that is lower than that of the images formed by dye inks.
An attempt to decrease the size of pigment particles has been made to improve the color developing ability of pigment inks. Pigments reduced in size to less than 100 nm (referred to hereinbelow as fine pigment particles) generally produce little light reflection, and have an increased specific surface area. Therefore, they can often have color developing ability on par with that of dyes.
A dispersing machine such as a sand mill, a roll mill, and a ball mill or an ultrasonic dispersing machine, is typically used for finely dispersing pigment particles to a relatively small primary particle size (100 nm or less).
However, in this process, new active centers may be formed by the destruction of the crystal shape of pigment particles and grinding of the pigment particle surface, and these active centers can degrade the stability of dispersion with time.
Accordingly, Japanese Patent Publication No. 6-96679 recently suggested a method for manufacturing fine pigment particles by which a pigment is dissolved in a solvent, and then the obtained solution of the pigment (referred to hereinbelow as the “pigment solution”) is mixed with a poor solvent of the pigment in the presence of a dispersant to cause a reprecipitation.
This method will be also referred to hereinbelow as a reprecipitation method.
The reprecipitation method provides improvements over fine powdering by an ultrasonic dispersing method or a dispersing method using a dispersing machine such as a sand mill, a roll mill, or a ball mill.
However, an aggregation force acting between the fine particles may tend to increase with the increase in the specific surface area of the particles occurring when the particles are finely ground (e.g., to a nanometer size particles).
In other words, where a fine particle dispersion is fabricated under high-concentration conditions, the aggregation of fine particles may significantly increase.
Methods for manufacturing pigment particles by using a reprecipitation method are described below.
Japanese Patent Laid-open No. 9-221616 suggests producing fine particles by an acid pasting method in which a pigment is temporarily dissolved using sulfuric acid. However, such a method typically cannot produce pigment fine particles with a size equal to or less than 100 nm.
U.S. Pat. No. 4,734,137 describes an organic pigment and a dispersant such as a surfactant or a resin that are together dissolved in an aprotic polar solvent in the presence of an alkali. The pigment is then precipitated by a reprecipitation method to obtain fine particles of organic pigment with relatively high dispersivity.
However, because this method involves dropwise adding of an acid to an organic pigment solution, and precipitating the organic pigment, the solvent is separated at the same time due to neutralization and reprecipitation.
As a result, the aggregation of pigment particles cannot be entirely prevented and a pigment of a nanometer size with an arranged particle size cannot be obtained, even when a dispersing treatment with a ball mill or the like is thereafter performed.
Thus, according to these methods, a fine particle dispersion with a high pigment concentration (for example, equal to or higher than 3 wt. %) cannot be easily obtained by merely mixing an organic pigment solution with a poor solvent in the presence of a standard dispersant.
However, where a high-concentration dispersion can be produced, it is believed that a significant increase in productivity can be achieved due to the reduction of the concentration process of dispersion and decrease in the amount of solvent used. Accordingly, there remains a strong demand for the development of a manufacturing method that can produce fine pigment particles dispersed with good stability under high pigment concentration conditions with high productivity.